Microporous polyolefin membranes are used for battery separators for primary batteries and secondary batteries such as lithium ion secondary batteries, lithium-polymer secondary batteries, nickel-hydrogen secondary batteries, nickel-cadmium secondary batteries, nickel-zinc secondary batteries, silver-zinc secondary batteries, etc. When the microporous polyolefin membrane is used for battery separators, particularly lithium ion battery separators, its performance largely affects the properties, productivity and safety of batteries. Accordingly, the microporous polyolefin membrane is required to have suitable permeability, mechanical properties, dimensional stability, shutdown properties, meltdown properties, etc. As is known, it is desirable for the batteries to have a relatively low shutdown temperature and a relatively high meltdown temperature for improved battery safety properties, particularly for batteries exposed to high temperatures during under operating conditions. High separator permeability is desirable for high battery capacity. A separator with high mechanical strength is desirable for improved battery assembly and fabrication properties.
The optimization of material compositions, stretching conditions, heat treatment conditions, etc. was proposed to improve the properties of microporous polyolefin membranes. For example, JP6-240036A, discloses a microporous polyolefin membrane having a proper pore diameter and a sharp pore diameter distribution. The membrane is made of a polyethylene resin containing 1% or more by mass of ultra-high-molecular-weight polyethylene having a weight-average molecular weight (Mw) of 7×105 or more, the polyethylene resin having a molecular weight distribution (weight-average molecular weight/number-average molecular weight) of 10-300, and the microporous polyolefin membrane having a porosity of 35-95%, an average penetrating pore diameter of 0.05-0.2 μm, a rupture strength (15-mm width) of 0.2 kg or more, and a pore diameter distribution (maximum pore diameter/average penetrating pore diameter) of 1.5 or less.
WO 2000/20492 discloses a microporous polyolefin membrane having improved permeability. The membrane contains fine fibrils made of polyethylene having Mw of 5×105 or more or a composition containing such polyethylene. The microporous polyolefin membrane has an average pore diameter of 0.05-5 μm, and the percentage of lamellas at angles θ of 80-100° relative to a membrane surface being 40% or more in longitudinal and transverse cross sections.
In general, microporous membranes consisting essentially of polyethylene (i.e., they contain polyethylene only with no significant presence of other species) have low meltdown temperatures. Accordingly, proposals have been made to provide microporous membranes made of mixed resins of polyethylene and polypropylene, and multi-layer, microporous membranes having polyethylene layers and polypropylene layers.
WO 2005/113657 discloses a microporous polyolefin membrane having pore-closability, thermal rupture resistance, dimensional stability and high-temperature strength. The membrane is made of a polyolefin composition comprising (a) a polyethylene resin containing 8-60% by mass of a component having a molecular weight of 10,000 or less, and an Mw/Mn ratio of 11-100, wherein Mn is the number-average molecular weight of the polyethylene resin, and a viscosity-average molecular weight (Mv) of 100,000-1,000,000, and (b) polypropylene, the membrane having a porosity of 20-95%, and a heat shrinkage ratio of 10% or less at 100° C. This microporous polyolefin membrane is produced by extruding a melt-blend of the above polyolefin composition and a membrane-forming solvent through a die, stretching a gel-like sheet obtained by cooling, removing the membrane-forming solvent, and annealing the sheet.
WO 2004/089627 discloses a microporous polyolefin membrane made of polyethylene and polypropylene as indispensable components, which is constituted by two or more layers, the polypropylene content being more than 50% and 95% or less by mass in at least one surface layer, and the polyethylene content being 50-95% by mass in the entire membrane. The membrane has improved permeability, high-temperature strength and safety, as well as low shutdown temperature and high short-circuiting temperature.
JP7-216118A discloses a battery separator formed by a porous film comprising polyethylene and polypropylene as indispensable components and having two microporous layers with different polyethylene contents, the polyethylene content being 0-20% by weight in one microporous layer, 21-60% by weight in the other microporous layer, and 2-40% by weight in the overall film. The battery separator has improved shutdown-starting temperature and mechanical strength.
With respect to the properties of separators, not only permeability, mechanical strength, dimensional stability, shutdown properties and meltdown properties, but also properties related to battery productivity such as electrolytic solution absorption, and battery cyclability such as electrolytic solution retention have recently been given importance. Particularly electrodes for lithium ion batteries expand and shrink according to the intrusion and departure of lithium, and recent increase in battery capacity leads to larger expansion ratios. Because separators are compressed when the electrodes expand, the separators are required to suffer as little decrease as possible in electrolytic solution retention by compression. However, when the separators are provided with larger pore diameters to have improved electrolytic solution absorption, the electrolytic solution retention of the separators decrease. Battery separators disclosed in any of JP6-240036A, WO 2000/20492, WO 05/113657, WO 04/089627 and JP7-216118A are insufficient in electrolytic solution absorption and/or retention. Thus, microporous membranes for battery separators are desired to have well-balanced permeability, mechanical strength, meltdown properties, electrolytic solution absorption, and electrolytic solution retention.